Coating compositions for coating the interior of containers intended for the storage of foodstuffs and beverages have two principal tasks. On the one hand, they should protect the container material itself against aggressive components of the contents, such as acids of natural origin, in order to give the container a maximum lifetime. On the other hand, they should prevent contamination of the contents by the container material, for example, as a result of chemical reaction. Appropriate coatings thus ideally represent a chemically inert, impermeable barrier between contents and container.
To perform the stated tasks reliably the coatings must meet a series of requirements. They are required, for example, to have a good adhesion to steel, tinplate, aluminum and other conventional container materials, to be resistant to dilute acids (such as acetic acid, lactic acid, carbonic acid) and sulfur even at increased temperatures, to be resistant to pasteurization and sterilization and also to have a high degree of elasticity so as to withstand deformations of the container material, whether during production of the containers or, for example, by compression of the filled container, without suffering damage.
In addition, the coating compositions must not contain components which migrate into the contents and alter them in any way. To this end, appropriate coating compositions are subject to a restrictive, statutory regulatory framework; for example, in the Federal Republic of Germany by regulations of the Federal Health Board (BGA), or in the U.S.A. by 21 C.F.R. .sctn. 175.300.
In order to obtain a coating film which is ideally inert, the coating compositions generally used are one-component systems, the reactive groups of which react fully at elevated temperatures within a very short time, for example, at 200.degree. C. in from eight to ten minutes, to form a high-grade crosslinked film.
Binders that have proven suitable in the past are relatively high molecular weight epoxy resins. In combination with appropriate curing agents, e.g., phenolic resins, amino resins, melamine and/or guanamine resins, dicyandiamide, polycarboxylic acids or their anhydrides, these epoxy resins cure fully under the stated conditions to give chemical-resistant, flexible films and are, in addition, permitted by the statutory regulations mentioned for use in the interior coatings of foodstuffs containers.
Coating combinations of this kind generally contain, however, a relatively large proportion of organic solvents. As part of the ever stricter statutory requirements with regard to the reduction or total avoidance of solvent emissions, one example of this being the VOC regulations in the U.S.A., the demand for low-solvent or solvent-free aqueous coating compositions for the interior coating of cans is growing. In fact there has already been a series of proposals in this direction, for the formulation of aqueous binders based on epoxy resin for the interior coating of foodstuffs containers.
One of the many possibilities proposed for the preparation of aqueous binders for the interior coating of cans is the modification of epoxy resins with acrylates. This modification may either be carried out via the reaction of epoxide groups with (meth)acrylic acid or derivatives thereof, in other words terminally, or by lateral grafting onto the epoxy resin structure. In almost all cases the monomers used include unsaturated acids such as, for example, acrylic acid, which are subsequently neutralized in order to impart solubility or dispersibility in water. Such systems generally still contain up to 25% of organic solvents and volatile amines and are of poor water-resistance.
"Acrylic systems" refers here to the systems obtained by (co)polymerization of .alpha..beta.-unsaturated monomers ("acrylic monomers"), such as acrylic, methacrylic and vinyl compounds but also maleic, itaconic and fumaric acid derivatives.
It has been shown that combinations of such acrylic systems with epoxy resins may have outstanding properties, which are determined by the advantageous properties of the individual systems. Thus the epoxy system contributes good adhesion, flexibility, chemical resistance and toughness, while the acrylic system, depending on the monomers chosen, provides the possibility of targeted adjustment of glass transition temperatures, hardness and mechanical resistance.
Numerous heat-curing epoxy-acrylic systems for the can interior coating sector have already been described, principally systems in which the epoxy resin has been induced to undergo specific grafting or esterification with the acrylic resin, and dispersion is carried out by neutralizing the carboxyl groups with volatile bases.
U.S. Pat. No. 4,302,373 describes a binder which is obtained from modified epoxy resins, acidic acrylic systems and tertiary amines, which has a low solids content and contains a lot of organic solvent. This composition is not sterilization-resistant and is therefore suitable only for beverage cans.
U.S. Pat. No. 4,285,847 describes a system obtained by bulk grafting of an epoxy resin with a monomer mixture, the acrylic system containing carboxyl groups. The product mixture, comprising ungrafted epoxy resin, epoxy-acrylic system and acrylic polymer is dispersed in a water/solvent mixture by neutralization with volatile bases and is then subjected to emulsion polymerization with further acrylic monomers. The resulting ionic dispersion has a solids content of about 20%, of which about 45 parts comprise epoxy resin and 55 parts acrylate (of which 40 parts are styrene). The solvent content, at from 10 to 20%, is decidedly high; the binder is likewise suitable only for the interior coating of beverage cans.
The company Glidden (J. T. K. Woo et al., ACS Symp. Ser. No. 221, 283 (1983)) has a system on the market which likewise represents an amine-neutralized epoxy-acrylic graft copolymer with a high epoxy resin content, a low solids content (20%) and a high level of solvent (20%). It is employed in the beverage can sector as a pasteurization-resistant clearcoat.
Likewise, the system described in EP-A-0 144 872, which is based on amine-neutralized epoxy-acrylic esters in combination with epoxide phosphate esters, brings no significant improvement and, in particular, no sterilization resistance.
As a further development of the Glidden system, EP-A-0 164 589 relates to an amine-neutralized dispersion based on epoxy-maleate-phosphate esters, in which the acrylic monomers are copolymerized at the double bond of the maleate radical and consequently no grafting occurs at aliphatic carbon atoms. The films obtained with this system are not pasteurization-resistant.
A dispersion of three-layer particles is described in EP-A-0 508 120. The particles are composed of a core of acrylates with a low glass transition temperature, a middle shell comprising epoxy resin, and an outer shell of acrylates of high glass transition temperature that have a high acid number. Copolymerization is carried out in bulk and the product is likewise dispersed using volatile amines, to give a solids content of 40% and a solvents content of 20%. This system too is suitable only for beverage cans.
A sterilization-resistant system is described in EP-A-0 130 686. An autocrosslinking, sterically stabilized acrylate dispersion is obtained by emulsion polymerization in water/alcohol mixtures, and a liquid epoxy resin is dispersed in this dispersion. No volatile amines are used in this case. The result is a dispersion for sterilization-resistant binders which, however, are used principally as a wash-coat for the exterior coating of cans rather than for interior coatings.
Other systems are known in which grafting between the epoxy resin and the acrylic system is induced non-specifically by bulk polymerization. EP-A-0 176 192 describes aqueous two-component epoxide systems for cold curing. Either the epoxy resin and acrylic monomers are mixed and dispersed and the acrylic monomers are then emulsion-polymerized, or the epoxy resin is dispersed alone and is blended with a ready-made acrylate dispersion. It is mainly liquid epoxy resins which are used; cold curing is carried out using the conventional amine systems. No mention is made of an application for heat-curing systems or for the can coating sector at all.
Finally, WO 89/01498 specifies a system in which the epoxy resin is partially esterified with methacrylic acid, dispersion is carried out by amine neutralization after blending with an acidic, acrylic prepolymer, and typically styrene is incorporated in the dispersion by emulsion polymerization. This ionic dispersion has a high polystyrene content (30-40% epoxy resin, 22-26% acidic acrylic prepolymer and 35-50% polystyrene) and is suitable for pasteurization-resistant coatings.
Generally, the systems described above, which represent the state of the art, are used only rarely for the interior coating of cans, since the use of volatile amines is a problem in terms of both toxicology and odor; ionically-stabilized binders generally have an insufficient storage stability which is further restricted by a rise in viscosity as a result of slow reaction of the coreactants (phenolic resins, melamine resins).
Previous attempts have failed to produce an autocrosslinking, nonionically-stabilized epoxy-acrylic dispersion which, with a high solids content and little or no solvent, leads to sterilization-resistant films and which meets the requirements of an interior can coating as mentioned at the outset.
One route to aqueous coating compositions is the use of nonionic emulsifiers or the introduction of hydrophilic, nonionic groups into the epoxy resin. In this way (see e.g., EP-A-0 272 595) aqueous dispersions can be prepared which are storage stable, free of amine and low in solvent. Using these dispersions and appropriate coreactants, such as phenolic resins and/or melamine resins, however, it has likewise not previously been possible to produce any can coating compositions the properties of which come close to the quality standard achieved by conventional, solvent-containing systems; for example, it was not possible to formulate white coatings free from yellowing.